1. Field of the Invention
The present invention relates to a dot area judging apparatus which is applied to an image forming apparatus such as a copying machine and adapted to determine whether or not any dot area is contained in a document original image read by a scanner or the like.
2. Description of Related Art
Digital copying machines are available which are adapted to optically read a document original image by a scanner such as comprised of a CCD (charge coupled device), convert the image into image data indicative of individual pixel densities, and reproduce the document original image on the basis of the image data.
Such a digital copying machine is designed to make a high quality copy of a document original even if a character image, a photographic image and a dotted image are contained together in the document original, the dotted image having shades represented by equidistantly arranged dots of various sizes. To this end, it is determined to which area type among a character area, a photographic area and a dot area each image portion in the document original belongs, and image data of the image portion are subjected to an image processing operation in accordance with the area type of the image portion. More specifically, pixels in the character area are subjected to an image processing operation such as a character edge emphasizing operation or a black character emphasizing operation. Pixels in the dot area are subjected to an image processing operation such as a smoothing operation for elimination of a moire.
Whether or not a particular image portion in the document original is a dot area is determined in the following manner. When the document original image is optically read and converted into image data, image data in a plurality of lines are retained in line memories. Then, a cross-shaped check area including an object pixel, two pixels located adjacent the object pixel along a primary scanning direction and two pixels located adjacent the object pixel along a secondary scanning direction is taken to determine whether or not the object pixel has a higher density than any of the adjacent pixels. If the object pixel has a higher density than any of the adjacent pixels, it is judged that the object pixel is a peak pixel. Conversely, if the object pixel has a lower density than any of the adjacent pixels, it is judged that the object pixel is a dip pixel.
After the peak/dip pixels in the check area are thus detected, it is determined whether or not an occurrence pattern of peak pixels or dip pixels in a judgment area composed of a predetermined number of pixels matches any of a plurality of mask patterns for dot areas, or whether or not the occurrence density of the peak pixels or the dip pixels is higher than a predetermined level. If there is a matched pattern or if the occurrence density is higher than the predetermined level, it is judged that the aforesaid judgment area is a dot area.
However, the aforesaid area judging method often fails to correctly judge a dot area in the document original image. Therefore, it is difficult to reliably detect dot areas.
It is therefore an object of the present invention to provide a dot area judging apparatus which is capable of reliably detecting a dot area.
It is another object of the present invention to provide a dot area judging method which is capable of reliably detecting a dot area.
It is a further object of the present invention to provide a image processing apparatus capable of properly performing an area adaptive processing on image data in a dot area.
The dot area judging apparatus according to the present invention comprises: a data retaining section for retaining image data indicative of densities of individual pixels in a plurality of lines in a document original image; a dot candidate pixel detecting section for detecting a dot candidate pixel on the basis of the image data retained in the data retaining section; a detection result retaining section for retaining detection results obtained for a plurality of lines by the dot candidate pixel detecting section; an extension section for extensively applying a dot candidate pixel detection result obtained for an object line by the dot candidate pixel detecting section to lines adjacent to the object line by modifying the detection results retained in the detection result retaining section; and a dot area judging section for judging, on the basis of distribution of dot candidate pixels within a predetermined judgment base area which distribution is obtained after the extension operation performed by the extension section, that a judgment area having a predetermined positional relationship with the judgment base area is a dot area.
With this arrangement, the dot candidate pixel detection result for the object line is extensively applied to the lines adjacent to the object line. Where an area on the object line is contained in the dot area, there is a high possibility that the lines adjacent to the object line are also contained in the dot area. Therefore, even if an erroneous judgment is made that pixels on the lines adjacent to the object line which are actually dot constituent pixels are not dot candidate pixels, the error can be corrected. By performing the dot area judging operation after the extension operation, the dot area can more reliably be determined than in the case where fixed mask patterns are employed.
In accordance with one embodiment of the present invention, the judgment base area to be employed for the judgment of the dot area includes as its constituent pixels an object pixel and a plurality of pixels located at vertices of a trapezoid surrounding the object pixel. Thus, even a sharp-pointed dot area can assuredly be detected.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.